Papers by Author: Hong Yi Jiang

Abstract: In this study, boron nitride (BN) fibers were prepared via chemical method using boric acid as the raw material, under the atmosphere of NH3 and N2, respectively. Firstly, the boric acid was heated to melt in order to acquire B2O3 fibers. Secondly, the B2O3 fibers were pre-azotized under a NH3 atmosphere and held for 1 h at 970°C, and the sample was then sintered in N2 to the temperature of 2000°C and held for 1 h. The obtained samples were texted to analyze nitrogen content, morphology, tensile strength, and elastic modulus by neutralization titration, SEM, XRD and so on. The results show that the diameter of the BN fibers is ~ 4-6 μm with the nitrogen content of 54%, tensile strength of 1400 MPa, and elastic modulus of 120 GPa.

Abstract: Silicon nitride coatings were prepared by chemical vapor deposition (CVD) method. The effect of raw precursor materials (SiH4 and NH3) ratio, gas flow amount, reaction temperature, and reaction pressure and deposition time to the microstructure, chemical compositions and crystal structures of the silicon nitride coatings are investigated. The results suggest that when the SiH4 and NH3 are used as the precursors, Ar is used as the protect gas, H2 is used as the carrier gas, the flow amount of SiH4 and NH3 is 200 sccm and 1000 sccm, the reaction pressure is 100 Pa, the reaction temperature is 900 oC and the deposition time is 30 min, the silicon nitride coatings with high deposition rate (~ 85 Å/min), small grain size (~ 0.2 µm) and high density are obtained. XRD results of the obtained silicon nitride coatings suggest their phase structure is amorphous.

Abstract: In this research, different amounts of LaCl3 were added to the lithium aluminum silicate (LAS) ceramics and the sample powder was prepared by sol-gel method. The effect of LaCl3 on the density, bending strength and the thermal expansion of the obtained ceramics was mainly investigated because LaCl3 is heavier than the lithium aluminum silicate (LAS) ceramics and the superfluous of La2O3 can be introduced into the sol solution.

Abstract: In this study, the porous silicon nitride ceramics with low permittivity was prepared by decomposition reaction of Si2N2O. The effect of sintering temperature on the properties of composites was investigated. The results show that the sintering process of porous silicon nitride ceramics is liquid crease-densititive-crease gas in high temperature-forming close pore-break of close pores and crease open pores. The density is 0.9-2.44 g/cm3, the porosity of sample varies from 0-47%, and the permittivity can be well controlled from 2.2 to 4.8.

Abstract: The single phase of Bi-doped Mg2Si0.5Sn0.5 compounds have been successfully fabricated by solid state reaction-spark plasma sintering (SPS). The effect of Bi doping concentration on the thermoelectric properties of Mg2Si0.5Sn0.5 is mainly investigated. The doping of Bi atom introduces impurity energy to Mg2Si0.5Sn0.5 compounds, which results in the increase of carrier concentration ( ), meanwhile it causes the increase of crystal distortion, enhancing the scatter of phonon. The results show that with the increasing of Bi doping content, the electrical conductivity (σ) increase, the absolute Seebeck coefficient ( ) and thermal conductivity ( ) decrease slightly in the measuring temperature range between 300 K and 800K. When the doping concentration of Bi is up to 2.5at% (nominal molar percent), the sample shows a maximum value of the figure of merit, ZT, is 0.78 at 800K.

Abstract: Nanocomposites and heavy doping both are regarded as effective way to improve materials’ thermoelectric properties. 0.7at% Bi-doped Mg2Si nanocomposites were prepared by spark plasma sintering. Results of thermoelectric properties tests show that the doping of Bi atom effectively improves the electrical conductivity of Mg2Si，and the nanocomposite structures are helpful to reduce thermal conductivity and increase Seebeck coefficient, hence improving the thermoelectric performance. A maximum dimensionless figure of merit of 0.8 is obtained for the Bi-doped Mg2Si nanocomposite with 50 wt % nanopowder inclusions at 823K, about 63% higher than that of Bi-doped Mg2Si sample without nanopowder inclusions and 119% higher than that of microsized Mg2Si sample without Bi-doped, respectively.

Abstract: Polycrystalline samples of Bi2Te3 based alloys were prepared by powder metallurgy processing including a melting-grinding and a sintering procedure of compacted pellets. Two sintering procedures as hot-pressing and spark plasma sintering (SPS) were employed. The thermoelectric properties and mechanical strength were measured in all case. Thermoelectric properties for p-type (Bi0.25Sb0.75)2Te3 and n-type Bi2(Te0.2Se0.8)3 changed with sintering temperature in both sintering methods. Mechanical strength and relative density increase with sintering temperature in two sintering procedures. The results firmly suggest that both sintering procedures are promising to obtain high performance thermoelectric materials.

Abstract: By two-step solid state reaction, Mg2Si0.8Ge0.2 was successfully synthesized .The effect of Ag-doping concentration on the thermoelectric transport properties of p-type Mg2Si0.8Ge0.2 was investigated. With the increasing of Ag-doping concentration, the electrical conductivity σ and Seebeck coefficient α increase correspondingly over the measuring temperature range (300-800K).The peaks of the curves of Seebeck coefficient versus temperature shift towards the lower temperature .When the doping concentration of Ag is 16000ppm(nominal molar percent), the power factor P of Mg2Si0.8Ge0.2 reaches 4.4×10-4W/mK2 at 800K,obviously improved in comparison with the sample with no additive .